1. Field of the Invention
The present invention relates generally to quartz pressure transducers for providing a variable output capacitance indicative of variable pneumatic pressure acting on the transducer, and more particularly to an improved pressure transducer having great resistance to temperature error while providing a highly accurate indication of pressure supplied to the transducer.
2. History of the Art
Quartz pressure transducers have been used for some time in the avionics field to convert a number of difference sampled pressures to electronic signals which may be analyzed to provide information about aircraft altitude, mach number, indicated airspeed, angle of attack, and slip or yaw angle. Since information on both military and commercial aircraft must be highly accurate, pressure transducers must be highly reliable and relatively immune from noise which may be caused by acceleration, turbulence, and temperature.
A system for measuring such pressures has three basic components, the first of which is a pitot tube which measures an air pressure at a location on the exterior of the aircraft, and has a pneumatic line leading from the pitot tube to the interior of the aircraft. The second component in the pressure measurement apparatus is the transducer which converts pressure into an electrical signal, with a typical transducer being of a fused quartz type such as that shown in U.S. Pat. No. 3,962,921, to Lips. Finally, the third component of the system is electronic circuitry which has as an input the variable capacitance from the quartz pressure transducer. Such circuitry is shown in U.S. Pat. No. 3,790,910, to McCormack.
Although several configurations of quartz pressure transducers are available, virtually all the designs are similar to the Lips patent design, and utilize two diaphragms with substantially equal natural frequencies, with the diaphragms being superposed for movement in phase and in unison under accelerating forces to minimize the effect of acceleration and vibration on the sensor. Ceramic and metalic transducers have also been made using the same principles as fused quartz transducers, and it should be understood that the principles of the invention disclosed herein may also be applicable to such transducers. More recently, a three element transducer has been manufactured by the assignee of the present application. By utilizing three elements, two sets of capacitor electrodes may be arranged, with one of the sets of capacitor electrodes being responsive to changes in pressure, and the other set of capacitor electrodes being immune to changes in pressure. Since both sets of capacitor electrodes are equally responsive to acceleration or vibration, the circuitry for the transducer may be so arranged and configured as to reduce the sensitivity of the assembly to acceleration and vibration error. This approach, while providing some degree of improvement in reduced sensitivity to acceleration and vibration, has not resulted in a reduction of error caused by temperature variation.
Stated succinctly, there are two causes of temperature error in quartz pressure transducers--thermal curvature of quartz diaphragms due to the presence of an electrode on only one side of the diaphragm, and differing physical characterics of the capacitor elements used in the pressure transducer. It is readily apparent that if only one side of a quartz diaphragm is plated with a deposited metal electrode, that diaphragm will bend slightly when subjected to temperature variation, in a manner similar to a bimetallic strip. Even more importantly, if one of the elements of the transducer is thicker than another, it is readily apparent that physical change with differing temperatures will not be consistent with the thinner element. Therefore, it is apparent that a need exists for a pressure transducer which will have substantially greater immunity to temperature variations. It is desirable that the improved transducer be so free of temperature error as to greatly reduce the requirement for temperature compensation in the electronic circuitry accompanying the transducer.
While the improved transducer has substantially increased immunity to temperature variation, it must also retain a high degree of immunity to vibration and acceleration forces. All of these desirable effects must be accomplished without degraduation or desensitization of the output of the pressure transducer. Finally, it is desirable that the improved transducer be adaptable for use both to sense absolute pressure and also to a differential pressure, whereby the transducer will detect the difference between two pressures applied to it.